GstStream: need API to deactivate streams

Submitted by Tim Müller @tpm

Link to original bug (#774321)

Description

+++ This bug was initially created as a clone of Bug 646638 +++

We now have new API to select and thus also deselect streams in form of the SELECT_STREAMS event.

We also want API that can be used to deactivate streams, which goes one step further, and would instruct the demuxer to not even read/demux/expose it.

This could be used for example in a thumbnailing application to deactivate decoding of all audio streams, or in GES which uses separate sources/decodebins for audio/video from the same input file.

Question is if we only want to be able to deactivate streams, or if we also want to allow re-activating them again later at runtime.